1. Field of the Invention
The present invention relates to a waveguide-photodetector including a waveguide and a photodetecting section integrally on a single substrate which is used in, for example, a magneto-optical information recording and reproduction apparatus, a method for producing the waveguide-photodetector, a waveguide having a tapered part usable in the waveguide-photodetector, and a method for producing such a waveguide.
2. Description of the Related Art
Magneto-optical disks have recently been a target of active research and development as rewritable high-density recording media. Information stored in such a magneto-optical disk is reproduced by detecting the rotation of the polarization direction of light reflected by the disk caused by the Kerr effect. In order to reproduce information stored in the magneto-optical disk with a satisfactory S/N ratio, a high-precision detector and a differential detection optical system and the like are necessary because the angle of the rotation of the polarization direction of light caused by the Kerr effect is effected on a microscopic level.
A conventional apparatus for reproducing information stored in a magneto-optical disk includes a bulk-type optical system, as an optical detection system, which includes a detector, a prism, a mirror, a lens and the like. In the bulk-type optical system, it is difficult to locate the optical elements in an appropriate positional relationship. Furthermore, the bulk-type optical system cannot be reduced in size and weight easily. In order to solve these problems, a waveguide-photodetector including a waveguide and a photodetecting section integrally on a single substrate has been proposed.
FIG. 15 is a schematic view of a conventional magneto-optical information recording and reproduction apparatus 90 including a waveguide-photodetector.
As shown in FIG. 15, the conventional magneto-optical information recording and reproduction apparatus 90 includes a light source 91 formed of a laser diode or the like, a light collection optical system including a collimator lens 93 and an objective lens 94 located so as to collect light from the light source 91 on a magneto-optical disk 92, a waveguide-photodetector 95 including a waveguide and a photodetecting section, and a prism coupler 96 provided on the waveguide-photodetector 95. The waveguide-photodetector 95 detects the light reflected by the magneto-optical disk 92. The prism coupler 96 is located on a light path between the collimator lens 93 and the objective lens 94. A bottom face of the prism coupler 96 reflects the light from the collimator lens 93 and directs the light toward the objective lens 94. The prism coupler 96 also guides the light reflected by the magneto-optical disk 92 and then transmitted through the objective lens 94 toward the waveguide-photodetector 95.
FIG. 16 is a plan view of the waveguide-photodetector 95.
In FIG. 16, a white area 97 is a first waveguide, and the prism coupler 96 is provided on the first waveguide 97. A mesh area 98 is a second waveguide, which is provided to be photocoupled to the first waveguide 97. Where the equivalent refractive index of the first waveguide 97 in the TE.sub.0 mode is Ne.sub.1, the equivalent refractive index of the first waveguide 97 in the TM.sub.0 mode is Nm.sub.1, the equivalent refractive index of the second waveguide 98 in the TE.sub.0 mode is Ne.sub.2, the equivalent refractive index of the second waveguide 98 in the TM.sub.0 mode is Nm.sub.2, Ne.sub.1 is substantially equal to Nm.sub.1, and the Ne.sub.2 is different from Nm.sub.2.
A hatched area 99 is a third waveguide which is provided in the second waveguide 98. Light propagated through the second waveguide 98 is divided into a light component in the TE mode and a light component in the TM mode at an interface between the second waveguide 98 and the third waveguide 99. The light component in the TE mode is reflected by the interface, and the light component in the TM mode is refracted by the interface. In other words, the third waveguide 99 acts as a mode separation element. As shown in FIG. 16, photodetectors 106 and 107 are respectively provided in the second waveguide 98 and the third waveguide 99 for detecting the light components in the TE mode and the TM mode, and the photodetectors 106 and 107 and the third waveguide 99 form a magneto-optical signal (MO signal) detector 108.
Photodetectors 101 and 102, and waveguide light collectors 103 and 104 for guiding the light to the photodetectors 101 and 102 are also provided on the second waveguide 98. The photodetectors 102 and 103 form a focus error signal (Fo signal) detector 105.
The conventional magneto-optical information recording and reproduction apparatus 90 operates in the following manner.
Light emitted by the light source 91 is collimated by the collimator lens 93 and is incident on the prism coupler 96. Then, the light is reflected by the bottom face of the prism coupler 96 toward the objective lens 94, and is collected on the magneto-optical disk 92 by the objective lens 94. The light reflected by the magneto-optical disk 92 is transmitted through the objective lens 94 and is incident on the prism counter 96 again. Then, the light is coupled to the first waveguide 97 in the waveguide-photodetector 95.
Thereafter, the light is propagated through the first waveguide 97, is coupled to the second waveguide 98, and is propagated through the second waveguide 98. Then, a part of the light is incident on the waveguide light collectors 103 and 104 and thus is guided to the photodetectors 101 and 102 to be used for detecting an Fo signal. The remaining part of the light is divided by the interface between the second waveguide 98 and the third waveguide 99 into a light component in the TE mode and a light component in the TM mode. The light components in the TE and TM modes are respectively guided to the photodetectors 106 and 107. Based on the outputs from the photodetectors 106 and 107, a magneto-optical signal is obtained.
With reference to FIG. 17A, the photodetectors 101, 102, 106 and 107 will be described.
FIG. 17A shows a cross-sectional view of an exemplary structure of a waveguide-photodetector including a waveguide and a photodetecting section on a single substrate. The photodetecting section corresponds to each of the photodetectors 101, 102, 106 and 107. In FIG. 17A, the waveguide-photodetector is indicated by reference numeral 110.
The waveguide-photodetector 110 includes an N.sup.+ -Si substrate 111 and an N.sup.- Si epitaxial layer 112 grown on the N.sup.+ -Si substrate 111. The N.sup.- Si epitaxial layer 112 includes a P.sup.+ region 112a formed by doping boron or the like into the N.sup.- Si epitaxial layer 112. The P.sup.+ region 112a acts as a part of a light receiving section of the photodetecting section.
A SiO.sub.2 layer 113 is provided on the N.sup.- Si epitaxial layer 112 as a result of thermal oxidation of the N.sup.- Si epitaxial layer 112. An area 113a of the SiO.sub.2 layer 113 which is on the P.sup.+ region 112a is thinner than the rest of the SiO.sub.2 layer 113. The SiO.sub.2 layer 113 acts as a buffer layer. On the SiO.sub.2 layer 113, a waveguide layer 114 formed of the first waveguide 97 or the second waveguide 98, or a waveguide layer 114' in which the first waveguide 97 and the second waveguide 98 are integrated is provided. In the following description, only the waveguide layer 114 will be mentioned for simplicity. An electrode interconnection 115 is provided on a part of the waveguide layer 114. The SiO.sub.2 layer 113 and the waveguide layer 114 have an opening 114a reaching the P.sup.+ region 112a. The electrode interconnection 115 is electrically connected to the P.sup.+ region 112a through the opening 114a. A gap layer 116 is provided on the waveguide layer 114 so as to cover the electrode interconnection 115. A rear electrode 117 is provided on a surface of the N.sup.+ --Si substrate 111 opposite the surface on which the N.sup.- Si epitaxial layer 112 is provided.
The waveguide-photodetector 110 having such a structure is generally produced in the following manner.
On the N.sup.+ --Si substrate 111 formed of single crystalline N.sup.+ silicon, the N.sup.- --Si layer 112 is epitaxially grown. A surface area of the N.sup.- --Si layer 112 is thermally oxidized to form the SiO.sub.2 layer 113 thereon. An area of the SiO.sub.2 layer 113 in which the light receiving section of the photodetecting section is to be formed is removed by etching or the like, thereby forming an opening. The resultant layers are kept in a high-temperature atmosphere including impurities such as boron, thereby diffusing the impurities into the N.sup.- --Si epitaxial layer 112 from the opening of the SiO.sub.2 layer 113. Thus, the P.sup.+ region 112a is formed. While the layers are kept in the high-temperature atmosphere, another SiO.sub.2 layer is formed, which is the area 113a thinner than the rest of the SiO.sub.2 layer 113 shown in FIG. 17A.
Next, on the SiO.sub.2 layer 113, the waveguide layer 114 formed of the first or second waveguide 97 or 98 is formed. In lieu of the waveguide layer 114, the waveguide layer 114' formed of the second and third waveguide layers 98 and 99 can be formed. As described above, the SiO.sub.2 layer 113 acting as a buffer layer has the area 113a which is thinner than the rest thereof. As can be appreciated from an inclining surface area 113b of the SiO.sub.2 layer 113, the thickness changes gradually in the vicinity of a light-incident side of the p.sup.+ region 112a (left end of the p.sup.+ region 112a in FIG. 17A). Namely, the SiO.sub.2 layer 113 has a tapered part on the light-incident side. In accordance with the tapered part of the SiO.sub.2 layer 113, the waveguide layer 114 formed on the SiO.sub.2 layer 113 also has a part inclining toward the p.sup.+ region 112a as a part of the light receiving section. The waveguide including the SiO.sub.2 layer 113 and the waveguide layer 114 has a tapered part on the light-incident side, which improves the detection efficiency of the waveguide-photodetector 110. The tapered part on the opposite side (right side in FIG. 17A) is not related to photodetection. Exemplary methods for forming such a waveguide having a tapered part (tapered waveguide) will be described later.
After the waveguide layer 114 is formed, the opening 114a is formed through the waveguide layer 114 and the SiO.sub.2 layer 113 so as to reach the P.sup.+ region 112a.
On the waveguide layer 114, the electrode interconnection 115 is formed by depositing a metal material or the like and patterning the material so as to fill the opening 114a. The gap layer 116 is formed on the waveguide layer 114 so as to cover the electrode interconnection 115. On the surface of the N.sup.+ --Si substrate 111 opposite the surface on which the N.sup.- --Si epitaxial layer 112 is provided, the rear electrode 117 is formed.
The light received by the light receiving section is converted into an electric signal corresponding to the amount of light by the photodetecting section and then is sent to an external device by the electrode interconnection 115.
As described above, the electrode interconnection 115, which is provided between the waveguide layer 114 and the gap layer 116, is electrically connected to the P.sup.+ region 112a acting as a part of the light receiving section, through the opening 114a formed through the waveguide layer 114 and the SiO.sub.2 layer 113 acting as a buffer layer. The gap layer 116 acts as an upper cladding layer for the waveguide layer 114 and also as a passivation layer for protecting the electrode interconnection 115 against shortcircuiting, mechanical damage, physical contamination, corrosion or the like. Since the electrode interconnection 115 is separated from the N.sup.+ --Si substrate 111 by the waveguide layer 114 as well as by the SiO.sub.2 layer 113, the capacitance between the electrode interconnection 115 and the N.sup.+ --Si substrate 111 can be lower than the capacitance in a structure in which the separation is performed only by the SiO.sub.2 layer 113.
FIG. 17B is a cross-sectional view showing another waveguide-photodetector 110'. In the structure shown in FIG. 17B, another gap layer 118 is provided between the electrode interconnection 115 and the waveguide layer 114. The electrode interconnection 115 is electrically connected to the P.sup.+ region 112a through the opening 114a formed through the gap layer 118, the waveguide layer 114 and the SiO.sub.2 layer 113 acting as a buffer layer. In such a structure, the gap layer 116 acts as a passivation layer for protecting the electrode interconnection 115 as described above, and the gap layer 118 contributes to reduction in capacitance between the electrode interconnection 115 and the N.sup.+ --Si substrate 111.
A tapered waveguide is useful for propagating the light in the thickness direction thereof with no loss or allowing the light to pass through the interface between two regions having different effective refractive indices with no loss. Hereinafter, the loss of light occurring during propagation in a waveguide layer will be referred to as the "propagation loss". The tapered part is formed by dry or wet etching, ion milling, mechanical processing such as cutting, or shadow masking. Shadow masking refers to covering an area of a layer and depositing another layer in a tapered manner using particles which jump to the covered area obliquely. Shadow masking can be used in known layer formation methods such as sputtering, vacuum evaporation, and CVD.
With reference to FIGS. 21A through 21I and 22, etching and shadow masking will be described. First, a method for forming a tapered part in a waveguide by etching disclosed in Japanese Laid-Open Patent Publication No. 4-55802 will be described with reference to FIGS. 21A through 21I.
A surface area of a Si substrate 191 shown in FIG. 21A is thermally oxidized to form a first SiO.sub.2 layer 192 thereon as shown in FIG. 21B. The first SiO.sub.2 layer 192 acts as a buffer layer. Next, as shown in FIG. 21C, a second SiO.sub.2 layer 193 is deposited on the first SiO.sub.2 layer 192 by spin coating. The second SiO.sub.2 layer 193 can be etched faster than the first SiO.sub.2 layer 192 formed by thermal oxidation. As shown in FIG. 21D, a photoresist 194 is formed on the second SiO.sub.2 layer 193 and patterned as prescribed as shown in FIG. 21E. Then, the lamination formed so far is etched with an appropriate etchant. As described above, the second SiO.sub.2 layer 193 is etched faster than the first SiO.sub.2 layer 192. Thus, as shown in FIG. 21F, an area of the first SiO.sub.2 layer 192 which is not covered with the photoresist 194 is etched away, and another area of the first SiO.sub.2 layer 192 which is covered with the photoresist 194 but not covered with the second SiO.sub.2 layer 193 is etched away in a tapered manner. The reason why the first SiO.sub.2 layer 192 is etched away in such a manner is that the first SiO.sub.2 layer 192 having a relatively low etching rate is etched away gradually and the first SiO.sub.2 layer 192 is etched away in proportion to the period of time of exposure to the etchant. Then, shown in FIGS. 21G and 21H, the photoresist 194 and the second SiO.sub.2 layer 193 are removed. As shown in FIG. 21I, a waveguide layer 195 is formed on the Si substrate 191 so as to cover the first SiO.sub.2 layer 192 acting as a buffer layer. In this manner, a waveguide having a tapered part is formed.
With reference to FIG. 22, shadow masking will be described. FIG. 22 shows a principle of shadow masking described in Journal of Lightwave Technology (vol. 8, No. 4, pp. 587-593, April 1990).
A metal mask 161 is distanced from a substrate 163 by a spacer 162. When a material for a layer 164 to be formed is deposited from a source (not shown) of the material located above the metal mask 161, the layer 164 is formed on the substrate 163, and furthermore particles of the material go into a space below the mask 161 because the particles jump obliquely as well as perpendicularly toward the substrate 163. Thus, the layer 164 has a tapered part 164a as shown in FIG. 22. The shape of the tapered part 164a depends on the shape of the horizontal cross-section of the mask 161, the distance between the mask 161 and the substrate 163, the size of the source of the material for the layer, and the distance between the source and the substrate 163.
FIG. 23 shows a method for forming a tapered part disclosed in Japanese Laid-Open Patent Publication No. 7-134216. This method uses shadow masking as described above.
A substrate 172 having a photoresist pattern 171 is located in an inclining posture between a sample table 176 and a source 175 of the material for the layer to be formed. The sample table 176 is cooled. Since the substrate 172 is distanced from the sample table 176 due to such a posture, a metal jig 174 having a satisfactory thermal conductivity is provided between the substrate 172 and the sample table 176 through a vacuum grease 173. For layer formation, particles 179 of the material are caused to jump at an angle with respect to a normal to the surface of the substrate 172, and thus the material is deposited on the substrate 172 so as to cover the photoresist pattern 171. In FIG. 23, an area of the material deposited on the photoresist pattern 171 is indicated by reference numeral 178, and an area of the material deposited directly on the substrate 172 is indicated by reference numeral 177. In an area of the substrate 172 shadowed by the photoresist pattern 171, the thickness of the material varies in correspondence with the degree to which the area is shadowed. After the material is deposited as prescribed, lift-off is performed. Specifically, the photoresist pattern 171 and the area 178 of the material, which is unnecessary, are removed by a solution such as acetone in which the photoresist can be dissolved. In this manner, the layer (area 177) having a prescribed tapered part is formed.
The above-described methods for forming the waveguide having a tapered part have the following problems.
Layer formation by vapor deposition, sputtering, CVD, or other types of deposition is advantageous over layer formation by thermal oxidation in that there is a wider range of selections for the material. However, the layer formed of a material selected from such a wide range allows grain boundaries to be generated more easily than the thermally oxidized SiO.sub.2 layer. Furthermore, the layer formed by any of the above-listed methods of deposition generally has a rough surface and is porous. The surface roughness of the tapered part is raised by etching. Due to these problems, a layer formed by any of these methods loses more light than a thermally oxidized SiO.sub.2 layer. Accordingly, the materials usable for forming a waveguide having a tapered part which performs properly are limited.
By the etching method described with reference to FIGS. 21A through 21I, when the second SiO.sub.2 layer 193 is removed, the area of the first SiO.sub.2 layer 192 which is not covered with the second SiO.sub.2 layer 193 is also etched away, resulting in generating a step portion in the first SiO.sub.2 layer 192. This causes propagation loss. Moreover, ions in the etchant may go into the substrate 191 or the substrate 191 may also be etched. Such influences on the substrate 191 may change the characteristics of the waveguide-photodetector.
The shadow masking method described with reference to FIG. 22 is disadvantageous both in reduction in size and integration of a photodetecting section and a waveguide. Since the mask 161 and the gap between the substrate 163 and the mask 161 are both about 1 mm, the length of the tapered part cannot be shorter than several millimeters. Furthermore, the steps of attaching and removing the mask 161 and the step of washing, for example, which are required for this method, make the mass-production of the waveguide-photodetectors difficult.
In the field of bulk-type optical systems, faster response and higher integration have been demanded for a photodetector due to the uses thereof. In this field, a photodetector is produced in parallel with ICs used in an external control circuit, and the structure of the photodetector is more complicated.
FIG. 18A is a cross-sectional view of a photodetector 120 to be produced in parallel with the ICs used in an external control circuit.
As shown in FIG. 18A, the photodetector 120 includes a p-type Si substrate 121, an n-type Si epitaxial layer 122 provided on the p-type Si substrate 121, a SiO.sub.2 layer 124 provided on the n-type Si epitaxial layer 122 as a result of thermal oxidation of a surface area thereof, metal layers 129, an anti-reflection layer 125 formed of a nitride, an insulative layer 126 formed of a nitride, and a passivation layer 127 for protecting an IC and metal wires (not shown). In FIG. 18A, a lead of the electrode interconnection is omitted for simplicity. The n-type Si epitaxial layer 122 has a P.sup.+ region 133 acting as a part of a light receiving section 128. The SiO.sub.2 layer 124 has an opening in correspondence with the P.sup.+ region 133, and a step portion 124a between the opening and the rest of the SiO.sub.2 layer 124 is micrometers high. The SiO.sub.2 layer 124 acts as a mask for diffusing impurities to form P.sup.+ regions 133. The metal layers 129 each act as an etching stopper and also as an interconnection for an IC.
The insulative layer 126, the passivation layer 127 and the metal layers 129 can be eliminated. The anti-reflection layer 125 is indispensable for the function thereof, and the SiO.sub.2 layer 124 is also indispensable for protecting the P-N junction. FIG. 18B is a cross-sectional view of a photodetector 120' having a simplest possible structure, in which the passivation layer 127, the metal layers 129 and the insulative layer 126 are eliminated. In even such a simple structure, a step portion 124a' which is about 1 .mu.m high is unavoidable. When the photodetector 120' is used in a bulk-type optical system, light is incident on the light receiving section 128 from outside (i.e., not through a waveguide). Accordingly, such a shallow step portion 124a' does not provide any problem.
With reference to FIGS. 19A through 19E, a method for producing photodetector 120 shown in FIG. 18A will be described together with the reason why a step portion 124a is unavoidably formed in the vicinity of the light receiving section in the photodetector 120 shown in FIG. 18A.
As shown in FIG. 19A, a surface area of the n-type Si epitaxial layer 122 grown on the p-type Si substrate 121 (FIG. 18A) is thermally oxidized to form the SiO.sub.2 layer 124b on the n-type Si epitaxial layer 122. The oxidation can be performed by dry oxidation or vapor oxidation. Dry oxidation refers to oxidizing the n-type Si epitaxial layer 122 in an oxygen flow. Vapor oxidation refers to oxidizing the n-type Si epitaxial layer 122 in an oxygen flow including vapor. An opening is formed in the SiO.sub.2 layer 124b as shown in FIG. 19B by, for example, forming a photoresist on the SiO.sub.2 layer 124b and patterning the SiO.sub.2 layer 124b.
Then, impurities are diffused into the n-type Si epitaxial layer 122 from the opening by, for example, keeping the resultant layers in a high-temperature atmosphere including the impurities using the SiO.sub.2 layer 124b as a mask. Thus, as shown in FIG. 19C, the P.sup.+ region 133 is formed. By keeping the n-type Si epitaxial layer 122 in the high-temperature atmosphere, the n-type Si epitaxial layer 122 is thermally oxidized again to form another SiO.sub.2 layer 124c also as shown in FIG. 19C.
In order to produce the photodetector 120 in parallel with the IC, a SiO.sub.2 layer 124d is formed on the SiO.sub.2 layer 124c and by CVD or the like as shown in FIG. 19D. The SiO.sub.2 layers 124b, 124c and 124d are shown as the SiO.sub.2 layer 124 in FIG. 19E. Then, as shown in FIG. 19E, an opening 134 is formed in the SiO.sub.2 layer 124 by etching. The opening 134 is inevitably smaller than the opening formed on the SiO.sub.2 layer 124b as shown in FIG. 19B by 2 to 3 .mu.m along the periphery thereof due to an insufficient positioning precision of masking or an insufficient etching precision. As a result, the step portion 124a is formed.
As can be appreciated from the above description, the step portion 124a is unavoidably formed in the vicinity of the light receiving section in the photodetector 120 shown in FIG. 18A having a structure suitable for faster response and higher integration. When the structure having the step portion 124a is used for a waveguide-photodetector described above designed for faster response and higher integration, an excessively complicated shape is required for the buffer layer in the vicinity of the step portion 124a. It is difficult to form such a complicated shape with high precision.
FIG. 20 is a cross-sectional view of a waveguide-photodetector 140 including a waveguide having a tapered part. The waveguide-photodetector 140 uses the structure described above with reference to FIG. 18A. With reference to FIG. 20, a method for such a waveguide-photodetector 140 will be described.
On the p-type Si substrate 141, an n-type Si layer 142 is epitaxially grown. A surface area of the n-type Si layer 142 is thermally oxidized to form a SiO.sub.2 layer 144 thereon. When an opening for forming a p.sup.+ impurity diffused region 153 is formed, a step portion H is undesirably generated as described referring to FIGS. 19A through 19E. An anti-reflection layer 145 is formed on the SiO.sub.2 layer 144, and a buffer layer 146 is formed thereon. In preparation for forming a waveguide layer 147 on the buffer layer 146, a top surface of the buffer layer 146 needs to be smoothed until the surface roughness thereof becomes sufficiently small to avoid an adverse influence on the propagation loss in the waveguide layer 147. Furthermore, the SiO.sub.2 layer 144 needs to be processed to have a shape (thickness, propagation length, inclination, etc.) so that there is substantially no propagation loss at a step portion H. If the top surface of the buffer layer 146 is not sufficiently smooth, a tapered part 147a of the waveguide layer 147 also obtains a step portion H'. When the buffer layer 146 is thinner than the design value in the vicinity of the step portion H', the light propagated from left to right in FIG. 20 runs from the waveguide layer 147 toward the p-type Si substrate 141 or to a space outside the waveguide opposite the substrate 141, thereby decreasing the photocoupling efficiency.
In the conventional waveguide-photodetectors 110 and 110' including a waveguide and a photodetecting section on a single substrate shown in FIGS. 17A and 17B, the surface of the impurity diffused region (P.sup.+ region 112a) acting as a part of the light receiving section of the photodetecting section is covered merely with, for example, the thin SiO.sub.2 layer 113. Accordingly, metal ions from outside, for example, alkaline ions may reach the impurity diffused region through the thin SiO.sub.2 layer 113 and exerts an adverse influence on, for example, the charge distribution at the P-N junction. Thus, the photodetecting performance of the waveguide-photodetector 110 may be degraded.
In such a structure, the impurity diffused region 112a acting as a part of the light receiving section of the photodetecting section is separated from the waveguide layer 114 also only by the thin SiO.sub.2 layer 113. Accordingly, when the waveguide layer 114 is formed of a glass material including metal ions, for example, #7059 glass (produced by Corning, Inc.), the metal ions reaching the impurity diffused region 112a through the SiO.sub.2 layer 113 may exert an adverse influence as described above. Optical, thermal, mechanical and electrical characteristics of glass materials may be varied by adjusting the composition thereof, and different methods of processing can be used for different compositions. However, since the metal ions in some glass materials have an adverse influence, glass materials which are usable for a waveguide-photodetector are limited.
When the waveguide layer is formed of a glass material, there are some formation methods which change the composition of the layer, make the layer porous or have other adverse influences on the resultant layer. Specifically, absorption of the light propagating in the waveguide layer by lack of oxygen and scattering of the light propagating in the waveguide layer due to an excessive surface roughness of the waveguide layer are closely related with propagation loss in the waveguide layer. Such a propagation loss can be reduced by annealing. For annealing, a temperature of as high as about 600.degree. C. is required, which is the softening point of glass. However, when the waveguide layer is formed and annealed after an electrode interconnection is formed of aluminum or the like, the electrode interconnection may be disconnected or oxidized by the heating. In such a case, the electrode does not function properly.
For forming the electrode so as to perform properly, a satisfactory precision is required for forming an opening in the waveguide layer, the buffer layer and the gap layer. In today's waveguide-photodetectors designed for faster response and higher integration, the size of the lead of the electrode interconnection is only several micrometers wide. It is difficult to form an opening having an aspect ratio of 1:1 in the waveguide layer, the buffer layer, and the gap layer at an appropriate position with respect to such a narrow lead.
A waveguide-photodetector including a waveguide layer, a buffer layer and a gap layer integrated on a single substrate has further problems of a change in characteristics and generation of cracks which may occur due to the stress generated in the layers.